Modulation system



June 2l,` 1949. w M, HALL 2,473,794

MoDULATIoN SYSTEM Filed June 22, i945 i762 2. ft 5.

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Patentedl June 2l, 1949 MoDULA'rIoN SYSTEM William M. Hall, Lexington,Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., acorporation of Delaware Application June 22, 1945, Serial No. 600,861

2 Claims. l

My present invention relates to magnetron oscillators, and moreparticularly to modulating systems therefor.

The magnetron oscillators lo which my present invention has specialreference generally consist of a cylindrical anode member, made ofhighly conductive material, such as copper, and provided with aplurality of inwardly-directed, radiallydisposed vanes, each pair ofadjacent vanes, together with that portion of said cylindrical anodemember lying therebetween, constituting a cavity resonator at thefrequency desired of the output of the device. Coaxial with saidcylindrical anode member is an indirectly-heated cathode membercomprising a suitable sleeve provided with a highly electron-emissivecoating, for

example, of the well known alkaline-earth metal oxide type. Means areincorporated inthe device for establishing a magnetic field in adirection transversely of the electron path between ,said cathode andanode members.

When a proper voltage is applied between the cathode and anode* membersof such a device, and the magnetic eld thereof is appropriately adjustedwith respect to the electric field created by said voltage,electromagnetic oscillations are generated having a wave lengthdetermined, primarily, by the geometry of the elements making up theaforementioned cavity resonators. Power is extracted from the device,for example, by

introducing a loop into one of said cavity resonators, said loopcoupling with the magnetic com ponents of said electromagneticoscillations.

Now, it is known to amplitude modulate such a magnetron oscillator byvarying to amplitude Y modulate such a magnetron oscillator by varyingthe voltage between the cathode and anode members thereof in accordancewith the modulating signal; but, inasmuch as the output voltage of suchan oscillator is a power function of the input signal thereto,considerable fdistortion is obtained, and relatively complex mechanismshave to be resorted to in order to compensate for such distortions.

It is, therefore, the main object of my present invention to accomplishsuch amplitude modulation without the introduction of distortion.

It is a further object of mv present invention to attain this highlydesirable result in a simple and inexpensive manner.

These, and other objects of my present invention, which will become moreapparent as the detailed description thereof progresses, are attained,briey, in the following manner:

If the product of the ratio of the voltage output 2 to the voltage onthe magnetron and the ratio of the voltage on the magnetron to thevoltage input is maintained constant, there is no distortion. In otherwords, the ratio of the voltage on the magnetron tothe voltage inputthereto must be the reciprocal of the ratio of the voltage output to thevoltage on the magnetron; and I have found that, because of thenon-linear rela.-

tionship between the input voltage and current tem of my presentinvention. It is, however, to

be clearly understood that I do not'wish to be limited to the detailsherein shown and described for purposes of illustration only, inasmuchas changes therein may be made without the exercise of invention andwithin the true spirit and scope of the claims hereto appended.

In said drawing,

Fig. 1 is a schematic diagram of a magnetron modulating system assembledin accordance withthe principles of my present invention;

Fig. 2 is a graph showing the relationship between the input and outputwith and without employing the expedients of my present invention; and

Fig. 3 is a graph explaining the manner in which my .present inventionoperates to correct for the distortions normally present in amplitudemodulating magnetron oscillators.

Referring now in more detail to the aforementioned illustrativeembodiment of my present invention, and with particular reference toFig. 1 of the drawing, the numeral 5 schematically indicates amagnetron, which, as set forth in earlier portions of thisspecification, may comprise a cylindrical anode member 6, provided witha plurality, here shown as eight, of anode vanes 1 arranged to define aplurality of cavity-resonators 8. Centrally disposed with respect to theanode member E is a suitable electron-emitting cathode member 9, andwhile, in the interests of clarity, it has been omitted from thedrawing, it will be understood that the device includes means forv 3establishing a magnetic ileld in a direction per pendicular to theelectron pat-h between the cathode member 9 and the anode vanes 1. Powermay be extracted from the device by means of a coupling loop I0.

'The anode and cathode members are connected, through conductors Il andI2, to a power supply for impressing a suitable D.C. voltagetherebetween, the anode conductor Il including the secondary winding I3of a transformer I4, whose primary winding may be connected to anappropriate source of modulating voltage, whereby the cathode-anodepotential may be varied to amplitude modulate the oscillating output ofthe device. However, with the device as thus far described, the voltageoutput varies with the voltage input as shown by the graph labelled Vocevs. Vm, Without Zo in Fig. 2 of the drawing, this relationshipresulting, of course, in considerable distortion.

By means of my present invention, I am able to obtain a relationshipsuch as is shown in the same figure by the graph labelled Voce vs. Vm.with Zo, a more nearly linear relationship which eliminates theaforementioned distortion.

I attain this relationship by inserting a series impedance Zo,preferably a resistor, intermediate the anode member B of the magnetronand the combined D.C. and A.C. voltage input thereto. Inasmuch as themagnetron presents a non-linear impedance to its input voltage, the dropacross the impedance Zo increases non-linearly with increasing inputvoltage, and the voltage actually applied to the magnetron varies in amanner, as shown by the graph labelled Vmag. vs Vm. with Zo in Fig. 3 ofthe drawing, inversely to the output voltage curve in the absence of theimpedance Zo, shown in Fig. 2 of the drawing.

In said Fig. 3, the dotted lines labelled IZo" indicate the increasingdrops across the impedance Zu, the sum of these drops and the voltageactually applied to the magnetron equalling the entire voltage input,labelled IZO-l-Vmg.

It will thus be seen that the ratio Vinum/Vin. can, by choosing theproper value for the imped ance Zo, be made the reciprocal of the ratioVoue/Vmag., the condition necessary to obtain a linear relationshipbetween the voltage output of the oscillator and the voltage inputthereto.

It might be pointed out that while the impedance Zo has here been shownto be an element in itself, said impedance may, if desired, consist,

for example. of the internal plate resistance of a modulator tube,multiplied, if necessary, by an appropriate factor through the use of asuitable output transformer.

This completes the description of my present invention. It will be noted-from all of the foregoing that I have provided simple and inexpensivemeans for enabling the modulation of a magnetron oscillator in suchmanner that the output voltage varies linearly with the input voltage,

whereby distortion is eliminated.

Other advantages of my present invention will readily occur to thoseskilled in the art to which the same relates.

What is claimed is:

l. In combination: a magnetron oscillator; a source of modulatingvoltage therefor; and means for rendering the output voltage of saidoscillator substantially linearly variable with said modulating voltage;said means comprising an impedance connected in series intermediate saidoscillator and said source of modulating voltage; said impedance beingof such value that the ratio of the voltage on said oscillator to saidmodulating voltage is approximately the reciprocal of the ratio of theoscillator output voltage to said modulating voltage in the absence ofsaid impedance.

2. In combination: a magnetron oscillator; a source of modulatingvoltage therefor; and means for rendering the output voltage of saidoscillator substantially linearly variable with said modulating voltage;said means comprising a resistor` connected in series intermediate saidoscillator and said source of modulating voltage; said resistor being ofsuch value that the ratio of the voltage on said oscillator to saidmodulating voltage is approximately the reciprocal of the ratio of theoscillator output voltage to said modulating voltage in the absence ofsaid resistor.

WILLIAM M. HALL.

REFERENCES CITED The following references are of record in the tile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,013,773 Welke Sept. 10, 19352,069,538 Philpott Feb. 2, 1937 2,201,666 Hollmann May 21, 19402,217,869 LeVan Oct. 15, 1940

